Fix typo

Signed-off-by: Adrien Gallouët <agallouet@gmail.com>

api/client/save.go

@@ -16,7 +16,7 @@ import (
 // Usage: docker save [OPTIONS] IMAGE [IMAGE...]
 func (cli *DockerCli) CmdSave(args ...string) error {
 	cmd := Cli.Subcmd("save", []string{"IMAGE [IMAGE...]"}, "Save an image(s) to a tar archive (streamed to STDOUT by default)", true)
-	outfile := cmd.String([]string{"o", "-output"}, "", "Write to an file, instead of STDOUT")
+	outfile := cmd.String([]string{"o", "-output"}, "", "Write to a file, instead of STDOUT")
 	cmd.Require(flag.Min, 1)
 
 	cmd.ParseFlags(args, true)